Chemical Reactions Chapter 7

Chemical Reactions A CHEMICAL REACTION HAS OCCURRED IF THE SUBSTANCES AT THE END OF THE REACTION ARE DIFFERENT FROM THE SUBSTANCES AT THE START OF THE REACTION

Why is it important to balance equations? What has to be true about the products and the reactants?

A Chemical Reaction Reactants Products

Writing and Balancing Equations A chemical equation is a shorthand expression for a chemical change or reaction. word - equation : states in words mercury (II) oxide + heat mercury + oxygen bulky and cumbersome 2 HgO 2Hg + O2

Writing and Balancing Equations General Format for Writing Equations 1. The reactants are separated from the products by an arrow that indicates the direction of the reaction. 2. The reactants are placed to the left and the products to the right of the arrow. A plus sign (+) is placed between reactants and between products when needed.

Writing and Balancing Equations 3. Conditions required to carry out the reaction may be placed above or below the arrow. A delta indicates heat is applied. 4. Coefficients 2 H2O are placed in front to balance the equation. One (1) is never placed there, it is just understood.

Writing and Balancing Equations 5. The physical state of a substance is indicated by the following symbols: (s) solid (l) liquid (g) gaseous (aq) aqueous: means water solution yields reversible reaction

Writing and Balancing Equations Atoms cannot be created nor destroyed by normal chemical reactions. This means we must have the same number and type of atoms in our products as we have in our reactants. Reactants Products

Matter Is Conserved H2 + Cl2 2 HCl + + Total atoms = Total atoms + + Total atoms = Total atoms 2 H, 2 Cl 2H, 2 Cl Total Mass = Total Mass 2(1.0) + 2(35.5) 2(36.5) 73.0 g = 73.0 g

Writing and Balancing Equations 1. First you must have the equation in symbol format, you cannot balance a word equation. 2. Balancing: a. Count and compare the number of atoms of each element on each side of the equation and determine those that must be balanced.

Writing and Balancing Equations b. Balance each element one at a time, by placing whole numbers (coefficients) in front of the formulas containing the unbalanced element. It is usually better to balance in this order: metals, nonmetals, hydrogen, oxygen.

Writing and Balancing Equations A coefficient placed before a formula multiplies every atom in the formula by that number! 2H2SO4 = 4H’s 2S’s 8O’s c. re-check each time you balance an element to see if anything else has become unbalanced. Make adjustments as needed.

Writing and Balancing Equations d. Do a final check making sure that each element is balanced and that the smallest possible set of whole number coefficients has been used: 4HgO 4Hg + 2O2 incorrect 2HgO 2Hg + O2 correct

P r a c t i c e 1 NaOH Na2O + H2O 2NaOH Na2O + H2O Fe + O2 Fe2O3 4Fe + 3O2 2Fe2O3

P r a c t i c e 2 O2 + H2 H2O O2 + 2H2 2H2O Cl2 + NaI NaCl + I2 Cl2 + 2NaI 2NaCl + I2

P r a c t i c e 3 CO2 + H2O C6H12O6 + O2 6CO2 + 6H2O C6H12O6 + 6O2 FeS + HCl FeCl2 + H2S FeS + 2HCl FeCl2 + H2S

Turn in Balancing Activity worksheet Balance the equations on the notes sheet.

P r a c t i c e 4 Al(NO3)3 + H2SO4 Al2(SO4)3 + HNO3

P r a c t i c e 5 Zn + HCl ZnCl2 + H2 Zn + 2HCl ZnCl2 + H2

P r a c t i c e 6 Al2O3 + H2O Al(OH)3 Al2O3 + 3H2O 2Al(OH)3

P r a c t i c e 7 Pb(NO3)2 + AlCl3 PbCl2 + Al(NO3)3

5 types of reaction 1. Double replacement reactions (DR) Acid base reactions are classifed as one type of double replacement reactions 2. Single replacement reactions (SR) 3. Synthesis reactions 4. Decomposition reactions 5. Combustion reactions

Chemical Reactions 2Mg + O2 2MgO Types of Chemical Reactions 3. Synthesis Reaction: Two reactants combine to give one product. A + B AB 2Mg + O2 2MgO

Synthesis or combination reactions Synthesis reactions occur when two substances (generally elements) combine and form a compound. (Sometimes these are called combination or addition reactions.) reactant + reactant  1 product Basically: A + B  AB Example: 2H2 + O2  2H2O Example: C + O2  CO2

Practice Predict the products. Write and balance the following synthesis reaction equations. Sodium metal reacts with chlorine gas Solid Magnesium reacts with fluorine gas Aluminum metal reacts with fluorine gas

Chemical Reactions Types of Chemical Reactions 4. Decomposition: A single substance is decomposed or broken down to give two or more different substances: AB A + B 2PbO2 2PbO + O2 2 Na HCO3 Na2CO3 + H2O + CO2

Decomposition Reactions Decomposition reactions occur when a compound breaks up into the elements or in a few to simpler compounds 1 Reactant  Product + Product In general: AB  A + B Example: 2 H2O  2H2 + O2 Example: 2 HgO  2Hg + O2

Practice Predict the products. Then, write and balance the following decomposition reaction equations: Solid Lead (IV) oxide decomposes Aluminum nitride decomposes

Chemical Reactions Types of Chemical Reactions 2. Single - Displacement Reaction: One element reacts with a compound to take the place of one of the elements of that compound. A + BC B + AC Zn + 2HCl H2 + ZnCl2

Single Replacement Reactions Single Replacement Reactions occur when one element replaces another in a compound. A metal can replace a metal (+) OR a nonmetal can replace a nonmetal (-). element + compoundelement + compound A + BC  AC + B (if A is a metal) OR A + BC  BA + C (if A is a nonmetal) (remember the cation always goes first!)

Single Replacement Reactions Sodium chloride solid reacts with fluorine gas Aluminum metal reacts with aqueous copper (II) nitrate

Types of Chemical Reactions 1. Double - Displacement: Two compounds exchange partners with each other to produce two different compounds. AB + CD AD + CB NaCl + KNO3 NaNO3 + KCl

More double replacement Double Replacement Reactions occur when a metal replaces a metal in a compound and a nonmetal replaces a nonmetal in a compound reactant + reactant  product + product AB + CD  AD + CB

Double Replacement Reactions First and last ions go together + inside ions go together Example: AgNO3(aq) + NaCl(s)  AgCl(s) + NaNO3(aq) Another example: K2SO4(aq) + Ba(NO3)2(aq)  KNO3(aq) + BaSO4(s) 2

3 Driving Forces of a Double replacement reaction Formation of a gas Formation of a precipitate (ppt.) Formation of a molecular compound such as water

Types of Chemical Reactions 5. Combustion: A hydrocarbon reacts with oxygen to form carbon dioxide, water, and energy. CxHy + O2 H20 + CO2 C5H10 + O2 H20 + CO2

Combustion Example C5H12 + O2  CO2 + H2O Write the products and balance the following combustion reaction: C10H22 + O2  8 5 6

Practice Balance and identify the type of reaction for the following equations: Cl2 + NaBr NaCl + Br2 Ca3(PO4)2 + H2SO4 CaSO4 + H3PO4 C2H4 + O2 H20 + CO2 Na + Cl2 NaCl H2O H2 + O2

Practice Balance and identify the type of reaction for the following equations: Cl2 + 2NaBr 2NaCl + Br2 (single replacement) Ca3(PO4)2 + 3H2SO4 3CaSO4 + 2H3PO4 (double displacement) C2H4 + 3O2 2H20 + 2CO2 (combustion) 2Na + Cl2 2NaCl (synthesis or combination) 2H2O 2H2 + O2 (decomposition)

Types of Reactions There are five types of chemical reactions we have talked about: Synthesis reactions Decomposition reactions Single displacement reactions Double displacement reactions Combustion reactions You need to be able to identify the type of reaction and predict the product(s)

Steps to Writing Reactions Some steps for doing reactions Identify the type of reaction Predict the product(s) using the type of reaction as a model Balance it Don’t forget about the diatomic elements! (BrINClHOF) For example, Oxygen is O2 as an element.

Practice Predict the products. Balance the equation HCl(aq) + AgNO3(aq)  CaCl2(aq) + Na3PO4(aq)  Pb(NO3)2(aq) + BaCl2(aq)  FeCl3(aq) + NaOH(aq)  H2SO4(aq) + NaOH(aq)  KOH(aq) + CuSO4(aq) 

Mixed Practice State the type, predict the products, and balance the following reactions: BaCl2 + H2SO4  C6H12 + O2  Zn + CuSO4  Cs + Br2  FeCO3 